Carbapenem-type antibacterial agents are excellent antibacterials having strong antibacterial activity against a wide spectrum of bacteria ranging from gram-positive bacteria to gram-negative bacteria including Pseudomonas aeruginosa. Hence, new antibacterials of the carbapenem-type are being energetically developed in recent years. Although carbapenem derivatives having no substituent at the 1-position in the carbapenem backbone, such as thienamycin shown by formula (III): ##STR3## have drawbacks that they are chemically unstable at high concentrations and that they are readily metabolized by dehydropeptidase I, incorporation of a .beta.-configuration alkyl group at the 1-position improves the stability of such carbapenem derivatives and enables the derivatives to be used alone without the necessity of addition of a dehydropeptidase inhibitor thereto. Therefore, efforts are currently being made to develop 1.beta.-alkylcarbapenem-type antibacterials and also to develop methods of synthesizing 4-[(R)-1-carboxyalkyl]azetidin-2-one derivatives represented by formula (II.beta.): ##STR4## (wherein R.sup.3 represents a lower alkyl group and R.sup.4 represents a hydrogen atom or a hydroxyl-protective group), which derivatives can be used as intermediates for such antibacterials.
As synthetic methods for compounds (II.beta.) described above, many reports have been made. The most promising method of these is to alkylate a 4-acetoxyazetidin-2-one derivative represented by formula (IV): ##STR5## (wherein R.sup.4 has the meaning as defined above and Ac denotes an acetyl group) at the 4-position with any of various nucleophilic agents thereby to incorporate a side chain. With respect to this method, the following reports, for example, have been made: alkylation with a propionic acid ester enolate [C. U. Kim et al., Tetrahedron Lett., 28 (5) 507-510 (1987); T. Chiba et al., Chem. Lett., 1343-1346 (1985); and T. Shibata et al., Tetrahedron Lett., 26 (39) 4739-4742 (1985)]; alkylation with a propionimide enolate [Y. Nagao et al., J. Am. Chem. Soc., 108, 4673-4675 (1986); Yoshimitsu Nagao, Kayak (Chemistry), 42 (3) 190-196 (1987); L. M. Fuentes et al., J. Am. Chem. Soc., 108, 4675-4676 (1986); R. Deziel et al., Tetrahedron Lett., 27 (47) 5687-5690 (1986); and Y. Ito et al., Tetrahedron Lett ., 28 (52) 6625-6628 (1987)]; and alkylation with a propionic acid thiol ester enolate [M. Endo, Can. J. Chem., 65, 2140-2145 (1987); C. U. Kim et al., Tetrahedron Lett., 28 (5) 507-510 (1987); and A. Hartei et al., Can. J. Chem., 66, 1537-1539 (1988)].
Other methods for synthesizing compounds (II.beta.) include, for example, a method of alkylating compound (V) ##STR6## (wherein R.sup.4 has the meaning as defined above) with lithium diisopropylamide [D. H. Shih et al., Heterocycles, 21 (1) 29-40 (1984)] and a method in which the exo-methylene group of compound (VI) ##STR7## (wherein R.sup.4 has the meaning as defined above, R.sup.5 represents a hydrogen atom or an amino-protective group, and R.sup.6 represents. an alkyl group, a carboxyl group, or an alkoxycarbonyl group) is reduced by catalytic reduction or by asymmetric reduction using a specific catalyst [JP-A-58-26887 (corresponding to European Patent 71908B); C. U. Kim et al., Tetrahedron Lett., 28 (5) 507-510 (1987); T. Ohta et al., J. Org. Chem., 3176-3178 (1987); and T. Iimori et al., Tetrahedron Lett., 27 (19) 2149-2152 (1986)]. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) These methods are reported in Yoshio Itoh et al., Yuki Gosei Kagaku (Chemistry of Organic Syntheses), 47 (7) 606-618, "Synthesis of the 1.beta.-Methylcarbapenem Key Intermediates"(1989).
According to these methods, compound (II.beta.) in most: cases is obtained in the form of compound (II) ##STR8## (wherein R.sup.3 and R.sup.4 each has the meaning as defined above) which is a mixture, in a specific proportion, of the compound (II.beta.) and compound (II.alpha.) ##STR9## (wherein R.sup.3 and R.sup.4 each has the meaning as defined above) which is a stereoisomer with the compound (II.beta.). This compound (II.alpha.) having an .alpha.-configuration alkyl group can be converted to the desired compound (II.beta.) having a .beta.-configuration alkyl group by isomerization, which may be conducted by the method disclosed, for example, in D. H. Shih et al., Heterocycles, 21 (1) 29-40 (1984).
However, the above-described methods for synthesizing compounds (II) and (II.beta.) are disadvantageous in that special and expensive reagents are used, reaction temperature extremely low, or expensive or toxic metals are used as catalyst. Therefore, the above methods are unsuited for syntheses in large quantities and are not being practiced on an industrial scale.
Hence, there has been a desire for development of process for efficiently producing compound (II), especially compound (II.beta.) which has a .beta.-configuration alkyl group and is more useful as an intermediate for 1.beta.-alkylcarbapenem-type antibacterials.